This invention relates to an image density correction device for correcting an image density difference in an image read from an original document by a CCD (a photoelectric converting element), and the method thereof, and an image reading device using the CCD, and an image forming apparatus such as an electronic copying machine which forms an image by using the image reading apparatus.
In the conventional image forming apparatus such as an electronic copying machine and a facsimile machine, pixel data transfer frequency by the image data reading device such as a CCD which reads an image from an original document sheet is increased as image data reading speed is increased. Accordingly, the control of the signals output from the CCD will become difficult. In order to ease the signal control, the pixels of the conventional CCD have been divided into two groups: odd and even pixels. In recent years, a quadruplex CCD in which the number of the pixel groups is increased by dividing the odd and even pixel groups respectively into first and second half pixel groups, i.e., front and rear pixel groups in the main scanning direction has been proposed. According to this proposal, the pixel data transfer frequency is decreased into a quarter of that of the CCD which does not divide pixels, to ease the processing of the signal.
In outputting image signals of the divided four pixel groups, however, the outputs of the four pixel groups respectively pass through the corresponding buffer amplifiers which are independent from each other. If the offset voltage of the four buffer amplifiers are different from each other, the image density of the read image may be affected thereby. For example, in the case where an image having a neutral tint (e.g. a gray chart) is read to precisely obtain fine difference in gradation, the offset voltages of the buffer amplifiers are respectively added to the voltage level of the original image density of the four pixel regions, with a result that the read image has four image densities.
This difference in image density is also caused by the CCD dividing the image data into odd and even pixel groups, of course. However, the odd and even pixels are alternately arranged, and thus the image density difference between the odd and even pixels is not clearly seen since the pixels are dispersed in a formed image in a marbled manner. While, in the image read by the above-mentioned quadruplex CCD, each of the odd and even pixels are divided into the front and rear pixel groups, and thus the formed image seems to be different in the image density in the front and rear portions of the image, and thus the image density difference is clearly seen. In order to solve this problem, the offset voltage correction by a staggered arrangement of a plurality of CCDs such as in the adhesion type sensor can be applied. According to this correction method, however, a gradation (gray chart) correction needs to be performed in addition to the conventional white and black shading corrections, and thus the algorithm of the correction operation becomes so complicated.